We report on the comparative study of the electromagnetic shielding effectiveness provided by different forms of nanocarbon dispersed in epoxy resin in low concentration (0.5 wt.%). A series of pre-percolative carbon black and close-to-percolation threshold carbon nanotube (CNT) samples were investigated in microwave frequencies in order to evaluate the shielding effectiveness of epoxy resin widely used for aerospace applications. The data of theoretical simulation on the basis of generalized Maxwell Garnett theory prove that the shielding effectiveness of the investigated composites in microwaves is determined mostly by the conductivity of nanocarbon inclusions. Consequently, utilizing well purified defect-free or chemically modified CNT we can improve drastically the shielding effectiveness of such a composite, without changing the volume fraction of nanocarbon inclusions. (C) 2011 Elsevier BM. All rights reserved.